Monophthongal is a linguistic term that refers to a vowel sound that is pronounced with a single, unchanging quality. It is the opposite of dipthongal, which describes a vowel sound that has two distinct qualities. Some possible antonyms for monophthongal might include "variegated," "multifaceted," "complex," "layered," "textured," or "nuanced." These words suggest that the sound in question is not simple or straightforward, but rather has many different elements or characteristics that make it difficult to describe or analyze. Other possible antonyms might include "unvaried," "uncomplicated," "uniform," or "homogeneous," which suggest that the sound is plain or basic, without any significant variation or complexity.